Changed add redis to use set with not existing flag

[argaen]

Closes #118

[codecov-io]

Codecov Report

Merging #119 into master will increase coverage by <.01%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
+ Coverage   98.85%   98.85%   +<.01%     
==========================================
  Files          12       12              
  Lines         609      610       +1     
  Branches       72       72              
==========================================
+ Hits          602      603       +1     
  Partials        7        7

Impacted Files	Coverage Δ
aiocache/backends/redis.py	98.76% <100%> (+0.01%)	✅

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